Something frivolous is, in today's context, either unimportant or not serious. Along with all the other Renaissance borrowings in the 15th century, this came from Latin, in this case the word frivolus. Frivolus carried more of the "unimportant" connotation but still had a side, metaphorical meaning of "silly". The meaning gets more focused as we travel backwards to frivos, which meant "crumbled", from friare, "to crumble". This connection exists because something unimportant breaks easily, and unimportant things are often silly, so what the hey. Friare apparently is related to a whole host of other Latin words, all of which derive from a reconstructed Proto-Indo-European root that sounded like bhreie and meant something super ubiquitous, like "cut" or "scrape".
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.